Two options on timing and scope of attacking France:
1. Start approach attacks on Benelux as soon as possible, starting with Luxembourg in October. Spend all MPPs on operating units west, existing upgrades and reinforcement of attacking troops. Occupy Netherlands and Belgium with the first troops that operate west from Poland, hopefully November or December. Plunge into France with whatever you have available. To paraphrase N.B.Forrest, get there the firstest with the mostest. Don't let the French have time to switch defenders around, entrench again and reinforce. Consider upgrading movement speed on any troops not attacking so that are better able to exploit any opportunities you develop.
2. Wait until April or May having first purchased more tanks, medium and tactical bombers. Your troops will be better, but the French position will be much improved.
"L'audace, l'audace, toujours l'audace."